Simanaitis Says

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QUICK: NAME a famous automaker who first manufactured aero engines.

At left, the Bavarian Coat of Arms. At right, BMW’s emblem.

Oh, the one with the spinning prop logo?

Actually, I was thinking of an even earlier one: W.O. Bentley and his B.R.1 rotary aero engine.

B.R.1, as in Bentley Rotary aero engine. This and another image from Jane’s All the World’s Aircraft, 1919, Arco Publishing reprint, 1969.

In 1912, W.O. (Walter Owen) and his brother H.M. (Horace Millner) established Bentley and Bentley to import French DPF cars into England. Later, during World War I, as a Lieutenant in the Royal Navy Air Service, W.O. accepted a challenge from the Admiralty to solve an aero engineering problem: The British were licensing production of the French Clerget 9B aero rotary, which powered, among other aircraft, the Sopwith Camel.

The Clerget 9B was a nine-cylinder rotary displacing 16.3 liters and producing 130 hp at 1250 rpm—and, as a rotary, this meant the entire engine, except for its fixed crankshaft, revved at this speed! The 9B weighed 381 lb., working out to 0.34 hp/lb., had a cost of £907 each, and tended to overheat.

Could W.O. beat the Clerget with a design of his own?

W.O. Bentley, 1888–1971, English engineer, co-founder of Bentley Motors Limited.

W.O.’s approach was both classic (he added displacement) as well as advanced (he swapped the Clerget’s cast-iron cylinders for aluminum ones fitted with pressed-in steel liners).

The B.R.1 paid homage to the Clerget in many ways: It too was a nine-cylinder powerplant relying on air cooling as its cylinders rotated at 1250 rpm. However, the B.R.1 produced 150 hp from its 17.3 liters, weighed only 16 lb. more than the Clerget’s 381 lb.—and cost £605 each.

Jane’s reported that the B.R.1 produced 0.38 hp/lb., versus the Clerget’s 0.34. W.O.’s design consumed fuel at a rate of 11.0 Imperial gallons/hour, compared with the Clerget 9B’s 12.0/hour.

Also, suggesting why WWI pilots never suffered constipation, both engines burned through about 12 Imperial pints/hour of lubricant, typically castor oil.

The B.R.1 became standardized for Sopwith Camels in the Royal Navy Air Service, though Bentley never achieved production capacity equal to demand, and most Royal Air Force Camels continued using the inferior, more expensive Clerget.

A Camel’s rotary engine in operation. Image from R&T, April 1967.

W.O.’s B.R.1 evolved into the B.R.2, displacing 24.9 liters, weighing 475 lb., by 1918 producing 250 hp (0.53 hp/lb.), and the last rotary engine used by the RAF. Later aero engines of this cylinder appearance were radials, not rotaries.

Bentley B.R.2 aero rotary engine, prop-mounting hardware at left.

With war’s end, in 1919 W.O. and H.M. committed to the automobile in establishing Bentley Motors Limited. Before long, W.O. would again show the French what for. This time, his designs would dominate 24-hour endurance racing in a place called Le Mans. ds

© Dennis Simanaitis, SimanaitisSays, 2017

5 comments on “B.R.1 ROTARY AERO ENGINE

  1. Skip
    September 10, 2017

    Dennis, this is a fascinating area to explore, thanks for the article. While not explicitly answering your leading questions, the following companies manufactured aero engines near the turn of the last century: Benz, Mercedes, Maybach, FIAT, Packard, Rolls Royce and even Sunbeam. A very fortunate development that BMW’s Max Friz was prohibited from designing aero engines after WWI. Up to mountains he went for a think, and down he came with the layout for the BMW bike that is still in production today. I had a vintage boxer in my formative years, and it was unforgettable.

  2. simanaitissays
    September 10, 2017

    Agreed, an interesting list. I’d add the Bugatti U-16 too,
    I’m fascinated by BMW’s and Bentley’s first designs being aero, not automotive.
    Interesting about Max Fritz. Much more fortuitous than the scrapping of the Zeppelin-Stakken E-4/20;

  3. Philippe de Lespinay
    September 10, 2017

    Let’s not forget two more quite important and relevant engine manufacturers, before and during WW1, the Gnome and Le Rhone companies, which produced rotary engines in fact superior to the Clerget 9-cylinder, but of which products were often shunned by bureaucrats in Paris. Clerget got contracts for 30000 of their engine, while Gnome and Le Rhone produced 25,000 of their 9-cylinder Delta and Le Rhône 110 hp rotary designs. Another 75,000 were produced under various licenses. These engines powered the majority of aircraft in the first half of the war, both Allied designs as well as German examples produced by Motorenfabrik Oberursel.The two companies will partner after WW1, their advanced pre-WW2 designs spied on and copied by the Japanese for Nakajima and Mitsubishi aircraft.

  4. David Northey
    December 7, 2018

    Excellent and interesting article. As Margaret Bentley’s grandson (W O’s step-grandson), I find your writings excellent.

    • simanaitissays
      December 7, 2018

      Thanks, David, for your kind words. A fine heritage, yours.

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